Succinylacetone (SA) is a specific irreversible inhibitor of delta-aminole-vulinic acid dehydrase (ALAD), the second enzyme of the heme pathway. ALAD from broken L1210 cells was inhibited completely by 1 micro mole SA. The compound was considerably less active against the enzyme in whole cells, but stopped cell growth at concentrations which only partially inhibited the enzyme. The rapid uptake of hematoporphyrin (HP) could be further enhanced by pretreatment of L1210 cells with SA. SA inhibited L1210 cells and this low rate could not be increased by incubation of the NIH3T3 cells with SA. Treatment of L1210 cells with SA for three days decreased the endogeous respiration by 50% and inhibited cell growth. An enhancer fragment from the long terminal repeat region derived from the Moloney sarcoma virus and an Alu sequence from a human source were cloned into the vector pi AN-7.